A physical quantity sensor such as an acceleration sensor and a yaw rate sensor detects physical quantity by measuring capacitance change. For example, a semiconductor acceleration sensor controls operation of an airbag system in a vehicle. The sensor includes a sensor chip having a movable electrode and a pair of fixed electrodes. The movable electrode is supported on a substrate with a spring so that the movable electrode is movable in accordance with acceleration applied to the sensor. Each fixed electrode is disposed apart from the movable electrode by a predetermined distance therebetween in a movable direction of the movable electrode. A carrier wave having a pulse shape is applied to each fixed electrode. Each carrier wave has a voltage, i.e., amplitude of 5 volts, and a phase, which is opposite to each other. A middle voltage of 2.5 volts is applied to the movable electrode. The acceleration is detected by measuring capacitance change between the movable electrode and the fixed electrode.
The acceleration sensor has a self diagnosis function for determining whether the sensor functions normally. For example, the sensor diagnoses whether a predetermined sensitivity is obtained or whether a foreign particle is inserted into a clearance of the sensor chip. The self diagnosis function is disclosed in, for example, U.S. Pat. No. 6,257,061. This function works in such a manner that the voltage to be applied to the movable electrode is switched from 2.5 volts to 4 volts when the sensor self diagnoses. By switching the voltage, strong electrostatic force is generated between one of the fixed electrodes and the movable electrode, so that the movable electrode is forcibly moved, i.e., displaced. The movement is converted into a voltage signal by a converter circuit in order to determine whether a predetermined voltage corresponding to the movement is obtained.
In the above function, the movable electrode is moved only to one side (e.g., to +X direction). Accordingly, the sensor cannot detect anomaly on the other side (i.e., on −X direction). In view of this point, another self diagnosis function is disclosed in U.S. Pat. No. 6,450,029. When the sensor self diagnoses, the frequency of the self diagnosis signal to be applied to the movable electrode is set in a range between a resonant frequency multiplied by 0.2 and the resonant frequency multiplied by 1.4. In this case, resonant magnification is equal to or larger than 1.0. Thus, the movable electrode is moved to each side; and therefore, the sensor can detect anomaly on both sides.
One of anomaly caused in the physical quantity sensor is sticking anomaly, which provides that the movable electrode contacts and adheres to the fixed electrode. The sticking anomaly is caused by an electrical factor of one electrode or a surface factor such as foreign particle or moisture on the surface of one electrode when excess acceleration is applied to the sensor or when the sensor is manufactured in a manufacturing process. The above self diagnosis function cannot detect the sticking anomaly.
Thus, it is required to determine whether the sticking anomaly is occurred. To determine whether the sticking anomaly is occurred (i.e., to perform a sticking diagnosis), the movable electrode forcibly contacts the fixed electrode. Then, the voltage is applied so as to separate the movable electrode from the fixed electrode. In this case, it is determined whether the movable electrode returns to a predetermined position. However, the forced displacement of the movable electrode becomes comparatively large when the movable electrode forcibly contacts. To obtain the large displacement, in general, a high voltage may be applied to the movable electrode.
Specifically, in case of the acceleration sensor for the airbag system, the sensor detects extremely high acceleration such as 50 G. Therefore, the sensor has a spring portion having high rigidity. The voltage for detecting the sticking anomaly is also extremely high, for example, over 10 volts. Thus, if a general 5 volts circuit is used for the sensor, it is necessary to add a booster circuit for generating a high voltage more than 10 volts by using a 5 volt power source. In this case, the construction of the circuit becomes complicated. Further, it is necessary to design a part of the circuit to withstand the high voltage, the part at which the high voltage is to be applied.
Thus, it is required for the physical quantity sensor to have self diagnosis function for the sticking anomaly with a simple construction.